The present invention relates to packaging and sealing an integrated circuit die.
To package a device in plastic, a typical die having an integrated circuit in a top face is bonded onto a paddle so that the bottom face of the die is bonded to the paddle. The die is then coupled to a leadframe with wirebonds. The die, paddle, and leadframe are encapsulated in plastic by flowing plastic over them at high temperature and pressure. This typical encapsulating process cannot be used with some dies, such as micromachined devices that have movable components, because the flowing plastic would affect the movement of the movable components.
Instead of packaging in plastic, some devices are packaged in hermetic containers to keep moisture away from the die. Several different types of packages are used, including a ceramic dual in-line package (cerdip), a ceramic side braze package, and a generally cylindrical metal package.
In such packages, the die is typically bonded to the inside of the package with a die attach, and then the package is sealed. A typical die attach in a cerdip process uses a silver paste. Such a paste can provide stress to a die, thus potentially degrading long-term performance.
According to the present invention, a cap is provided over part of a die so that at least part of the die is sealed and protected. The die can then be enclosed with flowing plastic which does not reach the sealed part of the die. While this approach has broad applicability, it is especially appropriate for micromachined devices that have a movable mass, a diaphragm, or a sensor that has a vibrating member. In the case of an accelerometer, the cap covers, surrounds, and seals the movable mass from flowing plastic. A cap can also be used to protect a die from other material, such as particulates. The cap can be a paddle bonded over (instead of under) a die, or can be a cap member that is provided in addition to a paddle. The seal can be just sufficient to protect from particles or plastic, or it can be a hermetic seal.
In a first embodiment of the present invention, a paddle that is smaller than a typical paddle serves as a die mount and also serves as a cap. A rear side of the paddle is bonded over the face of the die rather than under it with an adhesive that seals the desired part of the micromachined device sufficiently to prevent contact from flowing plastic. No adhesive is deposited in a center area of the paddle so that a space is defined between the die and the paddle. Thus, the adhesive also serves as a standoff to prevent the rear side of the paddle from contacting the face of the die. Alternatively or in addition to using an adhesive as a standoff, the standoff can also be achieved by forming a depression in the center area of the paddle surface by one of several methods, such as chemical etching and stamping.
In a second embodiment of the present invention, a rear side of a silicon cap is bonded to the face of the die to seal and protect the micromachined features from external material, such as flowing plastic. A cap made from silicon minimizes stresses due to mismatches in the thermal expansion and allows the use of standard fabrication techniques. The rear side of the silicon cap can be etched at central portions to define a cavity surrounded by a peripheral bond face. The cavity creates more space than the space created by the thickness of the adhesives to prevent contact between the cap and the face of the die, to reduce electrical interactions between the die and the cap, and to provide more volume to adsorb moisture. Alternatively, this central portion can be electrochemically etched to form porous silicon. Porous silicon structures have oxide surfaces that absorb moisture. Thus porous silicon etched into the internal surface of the cap will getter any moisture which may diffuse through the adhesive seal.
In a third embodiment of the present invention, a cap is bonded over the die so that it hermetically seals a portion of the die. If the die has a sensor, such as a suspended mass for sensing acceleration, the hermetic seal keeps moisture from the mass. To form the hermetic seal, a layer of glass or metal is provided over a top surface of the die, typically over a passivation layer (such as glass) with a perimeter that surrounds the portion of the die to be sealed. A cap, preferably made from silicon, has an etched central area and a peripheral area with a layer of glass or metal. This layer on the cap and the layer on the surface of the die are heated to a sufficient temperature so that the layers bond to create a hermetic seal.
In a preferred embodiment, the layer patterned over the passivation layer of the surface of the die is aluminum, and the layer around the perimeter of the cap is made of gold. An intermediate layer can first be provided on the die to assist bonding. After the portion of the die is sealed with the cap, the device can, if desired, be enclosed in plastic.
In each embodiment, the cap, whether a metal paddle or a silicon cap, is preferably kept at a constant voltage to shield the microstructure from electromagnetic noise, thus minimizing the susceptibility of the microstructure to electromagnetic fields from surface static charges in service or during manufacture. The cap can be tied to ground or to some other reference voltage on the die. The cap preferably has a smaller area than the die to allow access to bondpads located around the periphery of the die, thus enabling the use of standard wirebond equipment.
Accordingly, by bonding a cap over a die, portions of the die are sealed mechanically and electrically, thus allowing dies with movable masses, such as accelerometers, or dies with pressure sensors to be protected from external materials, such as plastic packaging. The cap does not add to the thickness of the packaged die; and if the cap is the paddle, the thickness of the package can be reduced.